Study on the high speed measurement of laser diode self-mixing velocimeter

Laser self-mixing interferometry is a new method proposed for measuring velocity of a moving target. When the optical beam is back-scattered into the active cavity, laser output power and frequency are modulated. By signal processing, the Doppler frequency can be acquired, and the target´s velocity can be calculated. Based on these properties, a low-cost interferometry velocity sensor can be designed. In order to know whether the Laser Diode (LD) Self-Mixing (SM) velocimeter is suitable for high speed (km/s) scene, the high speed measurement by LDSM is analyzed in theory and the simulation is processed. Based on the three-mirror cavity model, relations of the laser output power´s frequency fluctuations and Doppler frequency are acquired. External optical feedback is regarded as laser´s small-signal modulation by the Doppler frequency. From rate equations of the carrier density and the photon density, following conclusions is acquired: LDSM velocimeter´s maximum measurable velocity (MMV) is restricted by laser´s relaxation resonant frequency in principle; LDSM velocimeter´s MMV goes up to kilometers per second theoretically; When the LD´s modulation speed satisfies request, LDSM interferometry can be used to measure high speed and the measurement precision is very high. The result is available to use the self-mixing technique to develop high performance, small overall dimensions and low cast instruments to measure high speed.